The present invention relates to a reactor for the recovery of zinc contained in scrap, residues and mattes of that metal.
Due to the high level of demand for zinc, in particular for use as a means for protecting iron and steel by forming a covering thereron by galvanization, recovery of the zinc contained in scrap, residues and in particular galvanization mattes is becoming more important.
During the galvanization process, zinc-iron intermetallic components with occluded zinc are formed, due to the chemical reactions between the iron to be galvanized and the zinc bath, and direct attack of the bath on the galvanization tanks or containers. This includes the action of iron salts which are formed by virtue of the molten materials attacking the components to be galvanized. This produces a sediment at the bottom of the galvanization tanks or the components remain floating on the molten zinc, depending on the density thereof. The iron content of those compounds may be between 3 and 4% and even more.
Such compounds are commonly referred to as "mattes" or more correctly "bottom mattes" and "surface mattes" and form one of the more interesting residues, from the point of view of recovery of secondary zinc.
The process for recovering zinc from the above-mentioned mattes or residues is based on separation of the iron contained in the crystals of FeZn.sub.13 forming the matte, by means of the addition of aluminum to form an aluminum-iron intermetallic compound. That compound undergoes surface oxidation to be converted into a black powder which is not wetted by the molten zinc, thereby minimizing the losses in the separation operation.
The intermetallic compound formed is of a density which is lower than that of the molten zinc, whereby when the mixture is left at rest, it will float on the surface thereof. However, due to the viscosity of the liquid and the small size of the crystals, that operation will be found to be very slow.
In order to speed up the separation operation, it is known for the molten mass to be subjected to agitation in a reaction vessel which comprises a tank or vat for containing the mattes, scrap or residues in a molten state, with a paddle wheel disposed in the tank or vat and mounted on a vertical shaft. The shaft of the rotor member is supported by an upper head portion which is displaceable in a vertical direction. The upper portion also carries the motor for driving the shaft, and a cover for closing the tank or vat. In turn, the upper portion is supported by a horizontally displaceable carriage while the tank or vat is mounted on two external pivots which define an axis of rotation normal to the axis of the tank or vat, and permit it to be lowered.
In the reaction vessels of the above-indicated kind, the blades of the bladed rotor wheel are at a given angle for them to drive the molten mass towards the bottom of the tank or vat, thereby impinging against the bottom of the tank or vat and the reaction vessel walls. That arrangement of the blades of the bladed wheel produces, within the vat or tank, a flow of molten material in a closed circuit in a vertical direction, moving downwardly at the center of the material and upwardly at the periphery, with the flow being cut or interrupted by the blades of the agitator.
On the other hand, due to the configuration of the vat or tank, it is virtually impossible to achieve adequate control of the temperature of the molten material.
The above-described apparatus does not achieve the desired efficiency, essentially because of the characteristics of the flow of molten material which is generated within the tank, the impossibility of controlling the temperature within the tank or vat and the absence of control in respect to the air required for causing oxidation of the intermetallic compound.
The type of flow generated in the tank or vat and the lack of control in regard to the temperature of the material result in the formation of an intermetallic compound having the formula Al.sub.3 Fe, with a high melting point. That, combined with the lack of control in regard to the oxidation air, means that surface oxidation of the intermetallic compound and separation thereof from the molten material requires long apparatus operating times, including the necessity of stopping and starting the process a number of times.